UNDULATORY FORCES. LIUUT. 



[CIBCULAR POLARISATION. 



lattur class, when viewed umlur the polariacope, presents 

 intc rest in,- phenomena, ami of a kiuJ to which 

 ii foun-l uo analogy in any other department of 



If n niece of mi.-i be placed close to the analyser, and 

 carefully examined, a proper KjMi-imen will present two 

 iM.l which extend live series of rings, <>f 

 the most beautiful colours. These are best seen by 

 means of a tourmaline plato ; but the simple apparatus 

 which we have described answers the same purpose. 

 An addition of an eye-piece, which Mr. Bestall has made 

 to his polarisoope at our suggestion, shows these fine 

 effects exceedingly well. 



In the centra of the rings, the black spot indicates the 

 position of the optic axes of the mica ;* and besides the 

 rings and spots, a curved lino, cutting the oval rings, 

 may be observed. The annexed engraving gives a very 

 Fit-. 77. faint idea of the beatity of 



these effects; each of the 

 curves, of course, being co- 

 loured exactly like the New- 

 tonian rings, but far exceed- 

 ing them in size. A similar 

 effect is observed at both axes ; 

 and if the plate be revolved 

 round its centre, the curved 

 tand will assume a longitu- 

 dinal appearance. 



Nitre, which is also a 

 biaxial crystal, exhibits this phenomenon in a more perfect 

 manner, because the axes of its two crystals are close 

 to each other. A perfect crystal is to bo chosen, anil a 

 section made of it at right angles to its length, aii'l of 

 about the thickness of a sixth part of an inch. On this 

 being polished, and examined in the polariscope, the ap- 

 pearances presented are exceedingly beautiful. The two 

 axes, rings, <tc., are nearer together than in the case of 

 mica ; and their appearance is represented in the annexed 

 engraving, together with a cross, which extends between 

 the two axes.. The cross may be made to assume vari< nis 

 positions, by causing the piece 

 of nitre to revolve between the 

 analyser and the polariser, on 

 the geometrical axis of the 

 crystal. Particular attention 

 is required to the polish, thick- 

 . and other points. This, 

 and many other crystals, how- 

 ever, may be purchased, 

 ready mounted, of the op- 

 ticians. Amongst such, we 

 may mention Iceland spar, 

 which presents a very fine 

 appearance when of proper 

 thickness. 



S.>mr very beautiful effects 

 are observe. I when iinannealed 

 glass is viewed in the polari- 

 scopo. In our early remarks 

 on the influence of the mole- 

 jcular constitution of a mass 

 ^f on a ray of light p; 



through it, we stated, Hint 

 many liifi- renews could be noticed, by aM of pol.-r 

 which are entirely unpurceivel by common light. Glass, 

 whose parts are h"m"','en ooefibots; if, )mw- 



I, th 'ii its con- 

 stitution is altered, -, .vc . may be pore 

 therein. A piece of gUvw, a quarter of an inch thick, 

 ami an inch anil a -half square, if suddenly cooled from the 

 liquid mass, presents a very pr.-tty effect if placed some 

 distance from the analyser. This is represented in the 

 following engraving, which but crudely illustrates the 

 beautiful results produced. In tin- pic-,, from which we 

 have drawn our cut, each of the circular parts are dots 

 of a rich blue colour, separated by a white cross. The 

 latter changes to a black cross, with reddish-orange 

 on revolving the analyser through 00. 

 8e* (', p. 79. 



Fig. 78. 



This singular appearance and effect has a very valuublo 

 application. In the manu- 

 facture of glass, the greatest 

 care must be taken to have it 

 properly annealed before it is 

 introduced into commerce, if 

 the manufacturer has the least 

 regard to his own character 

 or the interests of his cus- 

 tomers. These considera- 

 tions, however, have very littlo 

 weight with those whose con- 

 sciences are in the inverse ratio 

 of their love of money; and 

 hence an immense quantity of glass vessels are annually 

 manufactured and sold, which, for the ordinary pur; 

 of life, are utterly valueless. Tliis arises from the 

 fact that they are badly annealed. The process of 

 annealing is intended to render glass less liable to break 

 on being subjected to sudden variations of tempe- 

 rature ; and is effected by first submitting the vessels 

 to a great heat, and then allowing them to cool very 

 gradually. Glass, like steel, may thus be rendered 

 more fit for daily use ; and, indeed, annealing and tem- 

 pering are similar processes, and produce similar results. 

 Well-annealed glass produces none of the appearances wo 

 have been describing ; whilst iinannealed readily answers 

 that purpose. If, therefore, any suspected glass vessel, 

 lens, <tc. , be examined by the polariscope, its character 

 may at once be ascertained. This fact is of considerable 

 use in daily life. To the philosopher it affords a means 

 of testing the nature of the glass of which his lenses and 

 chemical apparatus are made. To our housewives we may 

 equally offer the use of the polariscopo in the purchase 

 of glass vessels of any kind; and a judicious application 

 of the principles we have laid down, may have the double 

 effect of preventing annoyance by the sudden fracture of 

 badly annealed vessels, and an infraction on that patience 

 of which, it is said, the fairer sex are the chief possessors. 



CIRCULAR POLARISATION. 



O0R remarks have hitherto been confined to that kind 

 of polarisation which takes place in certain planes ex- 

 isting between the axes of any objects and the direction 

 of the polarised ray. We proceed now to notice briefly 

 some phenomena which are presented by solids and 

 liquids, in which we shall find that the effects depend 

 on a rotation, right or left reflectively, between the axis 

 of the body and the direction of the polarised light. 

 A plate or section of rock-crystal is an object wlu'ch 

 presents this peculiarity to the eye of the observer. 

 This is cut perpendicularly to its axis of refraction ; and 

 it affords, when viewed by means of an analyser 

 (which may bo a Nicol's prism), the appearance of rings 

 round its axes, the colours of which change according to 

 the direction in which the analyser is moved. The 

 colours succeed each other in regular order, just as we 

 find them presented by the prism :t and supposing the 

 analyser to be turned towards the RIGHT hand, then 

 ora n.^o, yellow, green, blue, and violet, successively 

 appear; and such bodies have what is called RIGIIT- 

 handed polarisation : if, however, these colours succeed 

 each other in any body when the analyser is turned 

 towards the LF.IT hand, then such is said to have a 

 i.EFT-handed polarisation. 



It may be as well, before we proceed further, to state, 

 that an ordinary polarising plate, and a N'i.-ol's prism, 

 will answer well for soli ient bodies, si; 



crystal, starch, <tc. ; but for fluids (of which we 

 shall presently speak), an arrangement is required of 

 a form suitable for the purpose ; one of which is re- 

 presented in the following engraving, and of the best 

 form we have tried. 



In Ki-,'. NO, n represents a Nicol's prism, fixed, by 



means of a rin^ and screw, on a common ring-stand ; l> 



is a tube in which the liquid is placed, which may 



be partly with black paper, to Keep out 



i 8w antf, p. 41. 



